1. Field of the Invention
The present invention relates to a device for recording/reproducing information in/from a record medium such as a DVD-RW (digital video disk rewritable or digital versatile disk rewritable) having a wobble on a track as phase-modulated address information, and relates to a demodulator circuit used in such a device for the purpose of demodulating addresses.
2. Description of the Related Art
In general, there is a need to correctly detect a linear velocity at each radial location of a record medium. To this end, many record media employ a format in which a track is wobbled at the time of manufacturing so as to produce a constant wobble-signal frequency when a CLV (constant linear velocity) control is engaged. A device for recording/reproducing information in/from a record medium detects this wobble signal, and uses the signal for the purpose of controlling rotation of the medium and generating a record clock signal.
Address information is also necessary for the purpose of identifying a record position in a record area where no information has been recorded. In this respect, Japanese Patent Laid-open Application No. 10-69646 discloses a method of phase-modulating track wobbles. Such a phase-modulation scheme may include several different methods. In any methods, however, there is a tradeoff between the amount of recorded information and S/N (signal to noise ratio) of the detected signal. Since a signal obtained from an optical-disk medium has low quality in general, a binary phase-shift keying scheme (BPSK or DPSK: binary modulation using 0xc2x0 and 180xc2x0 is the most suitable because it can achieve relatively high S/N.
Demodulation circuits for demodulating this phase-modulated address information (wobbling signal) include an analog demodulator circuit disclosed in Japanese Patent No. 6-19898 and a digital demodulator circuit disclosed in Japanese Patent Laid-open Application No. 5-260413. In these circuits, a carrier wave to which the phase information of a wobbling signal is superimposed is generated from the wobbling signal by a PLL or the like, and a phase difference (0xc2x0 and 180xc2x0 in the case of BPSK) is detected to demodulate the information. If a phase error develops between the original carrier wave of the wobbling signal and the carrier wave generated by the PLL, quality of phase demodulation drops, resulting in frequent detection errors.
Such methods and schemes as described above are generally used in the art of cameras and the field of communication. With respect to optical disks, the following problems need to be addressed.
Optical disks produce signals having low signal quality in an area where data has been recorded. To cope with this low signal quality, a wobbling signal is generally processed by a band-pass filter or the like before it is supplied to a PLL or the like to generate a carrier wave. Since such a filter is designed to remove phase modulation components and noise components effectively, a configuration having such a filter tends to frequently develop an inadvertent phase change (phase delay). Further, when access is made to the optical-disk medium under the conditions of constant angular velocity, the carrier-wave frequency of the wobbling signal varies depending on radial positions, so that the phase change (phase delay) caused by the filter as described above is easy to develop. Such a phase change serves as a phase error between the original carrier wave of the wobbling signal and the generated carrier wave, thereby resulting in deterioration of detection accuracy.
Accordingly, there is a need for a demodulation circuit, which can carry out accurate demodulation by properly detecting and correcting a phase error, and a need for a device for recording/reproducing information based on such a demodulation circuit.
Further, when address data is recorded by frequency modulating track wobbles, a carrier frequency of 22.05 kHz with a frequency shift of xc2x11 kHz is used to represent 0 and 1 of the data.
As previously described, Japanese Patent Laid-open Application No. 10-69646 discloses a method of phase-modulating track wobbles. Since a signal obtained from an optical-disk medium has low quality in general, a binary phase modulation scheme (BPSK or DPSK: binary modulation using 0xc2x0 and 180xc2x0) is most suitable because it can achieve relatively high S/N. This modulation scheme is employed in a system requiring a high S/N, so that more than one carrier-wave cycle is used to represent 1-bit data.
As a demodulation scheme, a scheme disclosed in Japanese Patent No. 6-19898 may be used, which teaches a demodulation device that extracts a carrier signal from a phase-modulated signal, and compares phases between the phase-modulated signal and the carrier signal to demodulate the modulated data. Alternatively, a typical phase-demodulation method that may be found in a text book may be used. A BPSK scheme, however, has a drawback in that the amount of information per unit time is smaller in comparison with the QPSK scheme or the like.
Address information on an optical disk is comprised of a synchronization signal representing a reference position of data and a data signal. It is generally desirable to be able to detect the synchronization signal without a need for special processing. Because of this reason, in a phase-modulation scheme, the synchronization signal is defined simply as a predetermined carrier-wave period that has a reversed phase relative to the carrier-wave signal. In order not to reduce the amount of information to be communicated, a carrier-wave period for the synchronization signal is desirably as short as possible.
The utmost requirement is that the data signal be able to be distinguished from the synchronization signal. Further, there is a need to avoid reducing the amount of information communicated via data signals when the data signals are provided in larger numbers than the synchronization signals. In consideration of the above, it is desirable that the data signals do not include a reversed-phase carrier-wave period longer than the synchronization signal, and rather include a shorter period of reversed phase.
When a phase is reversed during a short time period, however, a detected signal tends to have a relatively small signal level and become noisy, thereby more likely to generate errors.
Accordingly, there is a need for a demodulation circuit that can correctly demodulate data signals when phase reversal is applied to short carrier-wave periods, and a need for an information-recording/reproducing device based on such a demodulation circuit.
Moreover, the BPSK scheme has a problem in that the amount of information that can be communicated is small. To increase the efficiency as much as possible, the phase of a signal may be changed carrier-wave cycle by carrier-wave cycle. In this case, however, a related-art demodulation circuit may not have a sufficient demodulation capacity to cope with speed of such phase changes. Accordingly, there is a need for a demodulation circuit that can correctly demodulate signals, and a need for an information-recording/reproducing device based on such a demodulation circuit.
Accordingly, it is a general object of the present invention to provide a demodulation device and an information recording/reproducing device that can satisfy one or more of the shortcomings of the related art.
It is another and more specific object of the present invention to provide a demodulation device and an information recording/reproducing device that can carry out accurate demodulation by properly detecting and correcting a phase error.
It is yet another object of the present invention to provide a demodulation device and an information recording/reproducing device that can correctly demodulate data signals when phase reversal is applied to short carrier-wave periods.
It is yet another object of the present invention to provide a demodulation device and an information recording/reproducing device that can correctly demodulate signals.
In order to achieve some of the above objects, a device for demodulating position information that is recorded as track wobbling in a record medium by a BPSK (binary phase-shift keying) scheme includes a carrier-wave-generation circuit which derives a carrier wave from a wobbling signal obtained from the record medium, a phase-adjustment circuit which generates a phase-comparison signal having a 90xc2x0 phase difference relative to the carrier wave, a multiplier which multiplies the wobbling signal and the phase-comparison signal to supply a product signal, and a detection circuit which detects a phase error of the carrier wave introduced by the carrier-wave-generation circuit by detecting the phase error between the wobbling signal and the phase-comparison signal based on the product signal.
In the device described above, since the phase error is detected from the product of the wobbling signal and the phase-comparison signal having the 90xc2x0 phase difference relative to the carrier wave, the phase error between the carrier wave and the wobbling signal can be checked when such an error develops between the carrier-wave-detection system and the wobbling-signal-detection system in response to a frequency change or the like. Such a device is easy to implement especially when digital circuits are used.
According to one aspect of the present invention, the device as described above is such that the detection circuit includes a low-frequency-band-detection circuit which detects low-frequency components of the product signal to detect the phase error between the wobbling signal and the phase-comparison signal.
In the device described above, the phase error is detected by detecting low-frequency components of the product of the wobbling signal and the phase-comparison signal, so that a simple circuit configuration can perform the required task.
According to another aspect of the present invention, the device as described above is such that the detection circuit includes an integrator circuit which integrates the product signal over a time period to detect the phase error between the wobbling signal and the phase-comparison signal.
In the device described above, the phase error is detected by integrating the product of the wobbling signal and the phase-comparison signal, so that a simple circuit configuration can perform the required task, and can produce highly accurate detection results.
According to another aspect of the present invention, the device further includes an amplitude-detection circuit which detects an amplitude of the wobbling signal, and an amplitude-adjustment circuit which adjusts an amplitude of the phase-comparison signal according to the detected amplitude of the wobbling signal so as to attain a target amplitude.
In the device described above, stable phase-error detection can be made without use of an AGC circuit that serves to keep constant the amplitude of the wobbling signal.
According to another aspect of the present invention, the device described above is such that the phase-adjustment circuit adjusts the phase error to zero.
In the device described above, since phases are controlled such as to make the phase error zero, automatic tracking is achieved to track a difference in delay times between the wobbling signal and the carrier wave.
According to another aspect of the present invention, a device for demodulating position information that is recorded as track wobbling in a record medium by a BPSK scheme includes a carrier-wave-generation circuit which derives a carrier wave from a wobbling signal obtained from the record medium, a phase-adjustment circuit which generates a phase-comparison signal having no phase difference relative to the carrier wave, a subtraction circuit which obtains a difference between the wobbling signal and the phase-comparison signal to supply a difference signal, and a detection circuit which detects a phase error of the carrier wave introduced by the carrier-wave-generation circuit by detecting the phase error between the wobbling signal and the phase-comparison signal based the difference signal.
In the device described above, the phase error is detected by obtaining a difference between the wobbling signal and the phase-comparison signal that has the same phase as the carrier-wave signal, so that the phase error between the carrier wave and the wobbling signal can be checked when such an error develops between the carrier-wave-detection system and the wobbling-signal-detection system in response to a frequency change or the like. Further, there is no need for a circuit that generates a signal having a 90xc2x0 phase difference relative to the carrier wave, thereby avoiding an excessively large circuit structure because such a circuit is quite bulky when implemented as an analog circuit.
According to another aspect of the present invention, a device for demodulating position information that is recorded as track wobbling in a record medium by a BPSK scheme wherein a wobbling signal obtained from the record medium has a unit information thereof represented by a pair of a pre-phase-change portion and a post-phase-change portion, includes a carrier-wave-generation circuit which derives a carrier wave from the wobbling signal, an inversion-signal-generation circuit which generates an inversion signal indicative of a timing of a predetermined one of the pre-phase-change portion and the post-phase-change portion, a polarization-switch circuit which reverses a phase of the wobbling signal at the timing indicated by the inversion signal to generate a partially phase reversed wobbling signal, a synchronous-detection circuit which demodulates the partially phase reversed wobbling signal by comparing phases between the carrier wave and the partially phase reversed wobbling signal; and a data-demodulation circuit which demodulates the position information from an output of the synchronous-detection circuit.
In the device described above, the polarization of the wobbling signal is inverted for a predetermined portion of the unit information that has an opposite phase to the other portion of the unit information, so that the demodulation circuit is required to achieve less vigorous standards in terms of circuit speed for detecting phase changes, robustness against noise caused by synchronous detection around a phase change point, a capability to cope with a decrease in the output signal magnitude, etc. Therefore, a reliable demodulation circuit can be implemented by a simple circuit structure.
According to another aspect of the present invention, a device for demodulating position information that is recorded as track wobbling in a record medium by a BPSK scheme includes a carrier-wave-generation circuit which derives a carrier wave from a wobbling signal obtained from the record medium, a synchronous-detection circuit which demodulates the wobbling signal by comparing phases between the carrier wave and the wobbling signal, a plurality of integrator circuits which integrate an output of the synchronous-detection circuit over respective predetermined time periods, at least one sampling circuit which samples outputs of the integrator circuits successively, and a data-demodulation circuit which demodulates the position information from an output of said at least one sampling circuit.
In the device described above, the plurality of integrator circuits together with the sampling circuit operate in turn to continuously integrate and sample the results of phase comparison, which is made between the phase-modulated signal and the carrier-wave signal, thereby generating phase-demodulated data. This configuration makes it possible to detect the phase-modulation data under desirable conditions of S/N ratios, and, also, enables the integrator circuits to secure a sufficient discharge time.
According to another aspect of the present invention, the device as described above is such that the respective predetermined time periods are a multiple of one cycle of the carrier wave, and are variable.
In the device described above, reliable demodulation can be achieved by changing the periods of integration and the sampling timings depending on formats or types of information in a case such as when the number of carrier-wave cycles differs between the synchronization signal and the data signal.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.